1. Technical Field
This invention relates to power assisted rack and pinion steering systems for vehicles. More specifically, this invention relates to the connection between a torsion bar and a pinion in a rack and pinion steering gear assembly.
2. Prior Art
In a typical fluid power assisted steering system for a vehicle, a rotary valve directs the flow of hydraulic fluid under pressure to a hydraulic motor, and the motor moves steering linkage to effect turning of the steerable wheels of the vehicle. The rotary valve includes a valve core and a valve sleeve which are relatively rotatable. A steering input shaft extending from the hand wheel is formed as the valve core. One end of a torsion bar is rotationally fixed to the input shaft. An output member is rotationally fixed to the valve sleeve. The other end of the torsion bar is rotationally fixed to the output member.
When there is a resistive load on the output member from the steerable vehicle wheels, steering torque transmitted through the input shaft causes the torsion bar to twist. This action actuates the valve from a neutral condition and changes the rotary position of the valve core relative to the valve sleeve and, in turn, changes the fluid flow through the valve and to the motor. The motor moves the steering linkage to turn the steerable vehicle wheels. Steering linkage movement moves the output member in a follow-up manner to rotate the valve sleeve relative to the valve core and return the rotary valve to its neutral condition.
In rack and pinion steering gear assemblies, the pinion is the output member. Typically, one end of the torsion bar is first press fitted to the pinion. Next, the other end of the torsion bar is pinned to the input shaft by a drilling, reaming and pinning operation. Because the torsion bar is already rotationally fixed to the pinion, during this drilling, reaming and pinning operation the valve elements must be maintained in a hydraulically centered or balanced condition to avoid biasing the valve. The task of holding exact angular orientation between the torsion bar and the input shaft throughout the drilling, reaming and pinning operation is complex and requires expensive fixturing. Should angular movement or incorrect initial positioning occur, resulting in an unbalanced valve, the torsion bar is scrapped, increasing scrap costs and labor costs. Also, the drilling, reaming and pinning operation is conducted outside the final steering gear assembly because of the generation of metal chips during drilling and reaming.
There are various known means of connecting the torsion bar to the output member, including such methods as pinning and press fitting. Another method is shown in U.S. Pat. No. 4,285,266 which describes a worm and sector steering gear wherein tightening a nut against the end of the output member moves a tapered sleeve axially to clamp the torsion bar. In the sector gear steering assembly shown in U.S. Pat. No. 4,177,714, an end of the torsion bar is connected to a hub of the output member by a tapered sleeve received in a tapered bore in the hub.